A teenager accused of stabbing his sister to death with a kitchen knife in their Washington Township home last year made his first appearance in state Superior Court in Hackensack on Monday after a judge ruled last week that he could be tried as an adult.

Ian Silvera, a public defender, entered a not guilty plea on behalf of his client, Travis Gallo, who was 17 at the time of the fatal stabbing on the day after Christmas.

As Gallo’s adoptive parents and a brother watched from the gallery, Assistant Bergen County Prosecutor Danielle Grootenboer told Superior Court Judge Liliana S. DeAvila-Silebi that Gallo is charged in a criminal complaint with two counts: first-degree murder in the death of his 20-year-old sister, Teia Gallo, and possession of a weapon — a knife — for an unlawful purpose.

At the prosecutor’s request, DeAvila-Silebi continued the $2 million bail set on Thursday by Family Part Judge Gary N. Wilcox as well as the bail conditions barring Gallo from having telephone or mail contact with two of his siblings, James and Devon Gallo, or any other siblings who do not want to have contact with him.

“I’m not sure there’s any family member who doesn’t want to have contact with him,” Silvera countered, in asking the judge not to enforce that provision.

The prosecutor, however, argued the “no contact” order is “absolutely necessary in this case.” She said the investigation has documented prior “assaultive behavior” by Gallo against his siblings.

“Devon is a witness to the horrendous events that culminated in the death” of Teia Gallo, Grootenboer said. Gallo also assaulted James when he came to the defense of Teia during a prior incident in November, she said, adding that James took photos of his injuries after he was attacked.

The judge said the case will now proceed to the grand jury.

Contending that the bail is “too high,” Silvera told the judge he plans to ask that it be reduced at some point in the future. He said he also will request that Gallo be returned to the Bergen County Juvenile Detention Center in Teterboro because the County Jail “may be dangerous for him.”

“If he’s safe in the Bergen County Jail, I’m not going to consider him going to another facility,” the judge said.

The defendant’s father, Robert Gallo, an obstetric gynecologist, declined to speak with a reporter after the hearing, requesting privacy and saying, “We’ve been through a lot.”

The ruling transferring his son’s case out of juvenile court came as another blow to a family that already has experienced so much pain, he said last week.

The decision likely means a longer prison sentence for the teen if he’s convicted.

“Sentencing is significantly more lenient as a juvenile than as an adult,” Silvera said Monday, noting that as an adult, his client could face 30 years to life if convicted.

Asked whether Gallo might present a defense of mental disease or defect to avoid liability for the crime, he said “everything is on the table.” The decision about how the defense will proceed will be made after he has evaluated the evidence, he said.

Teia Gallo, a 2011 graduate of Paramus Catholic High School, was attending Dominican College in Orangeburg, N.Y., majoring in athletic training, at the time of her death.

Robert Gallo and his wife, Terrie, raised nine children of their own before they adopted Travis, Teia and Teia’s sister as babies, bringing the number of siblings to 12.